The stability of tryptophan and tryosine hydroxylases was studied in vitro. The activity of tryptophan hydroxylase decays rapidly when preincubated at 37 degrees C for up to 60 min. The inclusion of tetrahydrobiopterin (BH4), 6-methytetrahydropterin (6MPH4), or dimethyltetrahydropterin (DMPH4) in the preincubation mixture significantly protects tryptophan hydroxylase from 02-thermal inactivation. The protective effects were provided only be reduced cofactor since biopterin did not present inactivation. Tyrosine hydroxylase, on the other hand, is stable throughout a 60 min preincubation and the inclusion of BH4, 6MPH, or DMPH4 in the preincubation causes a loss of catalytic activity. The inhibitory substances is not a peroxide or superoxide anion, nor is it the cofactor or catecholamine end-product.